1. Field of the Invention
The present invention is directed to an improved electrofilter apparatus for accomplishing electrically augmented vacuum filtration to produce a dialyzed cake product.
2. Related Art
Electrically augmented vacuum processes and apparatus for carrying out dewatering of suspensions are disclosed in U.S. Pat. No. 4,107,026, issued Aug. 15, 1978, U.S. Pat. No. 4,170,529, issued Oct. 9, 1979, U.S. Pat. No. 4,207,158, issued June 10, 1980, U.S. Pat. No. 4,303,492, issued Dec. 1, 1981, copending and still pending application, U.S. Ser. No. 222,057, filed Jan. 2, 1981, U.S. Pat. No. 4,312,729, issued Jan. 26, 1982 and U.S. Pat. No. 4,419,209, issued Dec. 6, 1983. The entire disclosures of these patents and applications are incorporated herein by reference thereto.
In these earlier disclosures, self-contained hollow electrode assemblies are provided which are normally submerged in the suspension, but allow bodily removal therefrom for inspection or other purposes. These hollow electrode assemblies comprise two types of wall surfaces: ion-pervious walls for electrode assemblies of one polarity and liquid-pervious walls for electrode assemblies of the opposite polarity.
In operation, with electrode assemblies of both types immersed in the suspension, a source of vacuum is connected to the interior of the electrode assemblies having liquid-pervious walls to provide a controllable pressure differential, thereby producing a flow of carrier liquid through the filtration surface, while the solids migrate in the opposite direction, under the influence of the electric field, to deposit as cake upon the electrode assemblies having ion-pervious walls. Filtrate or permeate liquid, that is, carrier liquid freed of solids, is withdrawn or pumped from the interior of the liquid-filled hollow electrode structure at a controllable rate.
As indicated, cake deposition occurs on the hollow electrode assemblies having ion-pervious surfaces; these electrode assemblies are filled with an electrolyte and have an electrode element therein immersed in the electrolyte. These electrode elements are thus isolated from direct contact with the suspension. The electrolyte is specially selected for high conductivity and compatability with the electrode element. Since decomposition or evolution products and heat are generated at the electrode element within the hollow ion-pervious assembly, provision is made for a flow of electrolyte into and through the electrode chamber so that foreign products, including gases, and heat are swept out of the chamber and a relatively constant predetermined electrolyte composition is maintained.
Initially, the surface member employed at the anode was a filter cloth, such as a woven fabric made from Dynel (a copolymer of vinyl chloride and acylonitrile). The cake obtained with this arrangement was suitable for some commercial purposes, but was found to be ionically contaminated to an undesirable degree and to exhibit a high conductivity, which is disadvantageous. The contamination was found to be due in large measure to electroosmotic pumping of anolyte through the filter cloth into the cake. The remedy for this found in the prior art, was to substitute a hydraulically impermeable solid polymeric electrolyte or ion exchange membrane for the filter cloth at the anode. A cationic member was employed and this essentially eliminated contamination due to electroosmotic pumping. However, it was found that ionic contamination persisted, albeit at a lower level. It has now been determined that this lower level (but still undesirable) contamination is due to inherent characteristics of the cationic membrane at the anode.
There is a substantial advantage to be secured if this remaining contamination can be eliminated or greatly reduced in the electrically augmented vacuum filtration process. It is believed that the following description provides a solution to this contamination problem.